Process for the production of a heat-sealable oriented multilayer polyolefin film

ABSTRACT

An oriented multilayer polyolefin film which includes a base layer containing polypropylene, and at least one heat-sealable outer layer. In one embodiment, the base layer contains a tertiary aliphatic amine and an amide of a water-insoluble carboxylic acid having 8 to 24 carbon atoms and the heat-sealable outer layer contains SiO 2 . The multilayer film is essentially free from silicone oil and is useful, for example, as a packaging film.

This application is a division of application Ser. No. 08/200,145, filedFeb. 23, 1994, now U.S. Pat. No. 5,618,618.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to an oriented multilayer polyolefin filmcomprising a base layer containing polypropylene, and at least oneheat-sealable outer layer. The films are distinguished by a lowcoefficient of friction in combination with excellent heat-sealabilityand excellent processing properties.

2. Description of Related Art

The art describes various films having a low coefficient of friction.However, the demands made of the processing properties of films andtheir smooth passage through automatic machines have increasedconstantly over the years. For this reason, ever-lower coefficients offriction are demanded; today the term "low" friction values means in theorder of from 0.3 to 0.1, while a few years ago, a friction value offrom 0.4 to 0.5 was regarded as extremely low.

It is known from the art that the surface-slip characteristics ofpolyolefin films can be improved by adding a carboxamide. The filmsdescribed contain amides in the outer layers or in the base and outerlayers.

DE-A-2,001,032 describes films made from various thermoplastics whosesurface-slip characteristics have been improved by addition ofcarboxamides and antiblocking agents. Since sufficient amounts oflubricants cannot be incorporated into the outer layers alone, theadditional incorporation of the amides into the base layer isrecommended. These films have a coefficient of friction in the rangefrom 0.4 to 0.8 and thus no longer meet today's quality requirements.U.S. Pat. No. 4,117,193 describes multilayer films comprising apolypropylene base layer containing a lubricant, an antiblocking agentand an antistatic. The outer layer of these films comprises a polymerblend, and additionally contains a lubricant and an antiblocking agent.The polymer blend comprises an ethylene-butylene copolymer and apolyolefinic resin such as HDPE or LDPE. It is stated that the deficientsurface-slip characteristics of the films cannot be sufficientlyimproved by the addition of lubricants and antiblocking agents alone.For this reason, the outer layer is modified by addition of HDPE or LDPEin combination with a lubricant and antiblocking agent. According to theexamples and comparative examples, the reduction in the coefficient offriction is essentially due to the addition of HDPE. Pure copolymerouter layers having the same additive composition have coefficients offriction of from 0.7 to 0.8. The films combine excellent coefficients offriction with good printability, but the addition of thefriction-reducing polyolefinic resin means that they have veryunsatisfactory haze and gloss.

The art furthermore states that an excellent lubricant for achievingfriction values of less than 0.3 is the addition of silicone oil topolyolefinic films. Some publications recommend the use of the siliconeoil in combination with other lubricants.

EP-A-0,182,463 describes a multilayer film containing from 0.05 to 0.2%by weight of tertiary aliphatic amine in the base layer and acombination of silicone oil and SiO₂ in the heat-sealable outer layer.According to the description, the surprising interaction of SiO₂,silicone oil and amine in combination with a selected outer layerthickness of less than 0.8 μm gives films having a coefficient offriction of 0.3 or less. In spite of this excellent coefficient offriction, the processing properties of the film are deficient. Inparticular, it is not printable and is therefore unsuitable for manyapplications.

EP-A-0,143,130 discloses films containing a carboxamide in the baselayer and likewise the combination of silicone oil and SiO₂ in the outerlayer. As in EP-A-0,182,463 mentioned above, a synergistic effect of thethree selected components on the coefficient of friction is described.These films likewise have deficient processing properties, in spite oftheir advantageous surface slip. They too lack the important property ofprintability.

EP-A-0,194,588 and EP-A-0,217,598 describe a film which, in spite ofgood surface- slip characteristics, is readily printable in spite of theaddition of silicone oil. In these films, silicone oil, if desired incombination with SiO₂, is only incorporated into one outer layer. Thesecond, silicone oil-free outer layer is corona-treated to improve theprintability. The transfer of silicone oil onto the surface of thistreated second outer layer subsequently takes place by contact with thefirst, silicone oil-containing outer layer. This trick gives a siliconeoil-treated film having good surface-slip characteristics which issimultaneously readily printable on the corona-treated side, but isnevertheless heat-sealable.

This film has the disadvantage that it is only printable on one side.This is particularly disadvantageous for use of the film in thepackaging sector. Packaging frequently requires further labeling on itssecond surface, for example a bar code, and for this reason requires afilm having good surface-slip characteristics which is printable on bothsides. It has furthermore proven disadvantageous that the pretreatmentintensity of the corona-treated surface is no longer measureable due tothe siloxane. This means that it is difficult for the film processor tocheck the surface tension of the film, an important quality feature. Theprocessing properties of the film are also unsatisfactory.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide amultilayer polypropylene film which has a low coefficient of friction.The film should have better processing properties than known films. Afurther object is for the film to be heat-sealable, and to be printableon both sides. The pretreatment intensity should be easy for theprocessor to measure. However, these improvements should not impair theexternal appearance of the film, i.e., the film should simultaneouslyhave the lowest possible haze and high gloss.

It is further an object of the present invention to provide a method ofmaking such a film.

It is also an object to provide methods of using such films, such as inpackaging and laminates.

It is also an object of the present invention to provide a method forprocessing such films.

In accordance with these objectives, there has been provided, inaccordance with a first aspect of the present invention abiaxially-oriented multilayer polyolefin film comprising a base layercomprising a propylene polymer, a tertiary aliphatic amine, and an amideof a water-insoluble carboxylic acid having 8 to 24 carbon atoms, and aheat-sealable outer layer containing a heat-sealable olefin polymer andSiO₂, wherein the multilayer film is essentially free from silicone oil.

In accordance with another aspect of the present invention, there hasbeen provided a biaxially-oriented multilayer polyolefin film comprisinga base layer containing polypropylene, and at least one heat-sealableouter layer, wherein the multilayer film has a coefficient of frictionof from 0.12 to 0.3 after processing and contains essentially nosilicone oil.

In accordance with another aspect of the present invention, there hasbeen provided a process for the production of the multilayer polyolefinfilm described above, in which the melts corresponding to the individuallayers of the film are coextruded through a flat-film die, thecoextruded film is taken off via a take-off roll whose temperature isfrom 50° to 110° C., the film is biaxially stretched at a longitudinalstretching ratio of from 4:1 to 7:1 and a transverse stretching ratio offrom 8:1 to 10:1, the biaxially-stretched film is thermofixed,optionally corona-treated, and subsequently wound up, wherein the filmhas a coefficient of friction of from 0.2 to 0.3, after processing.

Further objects, features and advantages of the invention will becomeapparent from a review of the detailed description of the preferredembodiments which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The base layer of the multilayer film according to the inventionessentially comprises a propylene polymer and, if desired, additivesother than silicone oil, in effective amounts in each case. "Essentiallycomprises" means that the propylene polymer or a mixture of propylenepolymers is generally present in an amount of at least 50%, preferablyof at least 90% by weight, based on the weight of the base layer.

Any polymer from propylene can be used as the propylene polymer of thebase layer. In particular, the polypropylene polymer generally containsat least 50%, preferably at least 90% by weight propylene and generallyhas a melting point of 140° C. or above, preferably from 150° to 170° C.

Isotactic homopolypropylene having an n-heptane-soluble content of 6% byweight or less, based on the isotactic homopolypropylene, copolymers ofethylene and propylene having an ethylene content of 5% by weight orless, and copolymers of propylene with C₄ -C₈ -α-olefins having anα-olefin content of 5% by weight or less are preferred propylenepolymers for the base layer, particular preference being given toisotactic polypropylene. These percentages by weight relate, in eachcase, to the respective copolymer. The propylene polymer of the baselayer generally has a melt flow index of from 0.5 g/10 min to 10 g/10min, preferably from 1.5 g/10 min to 4 g/10 min, at 230° C. and a forceof 21.6 N (DIN 53 735).

Any desired heat-sealable outer layer can be used as long as it isessentially free of silicone oil. The outer layer generally contains oneor more heat-sealable olefin polymer, generally in amounts of at least50% by weight of the outer layer. Preferred heat-sealable olefinpolymers include a copolymer or terpolymer of propylene or a mixture oftwo or more such copolymers and/or terpolymers. The copolymers andterpolymers generally contain propylene as the principal component in anamount of at least 50% by weight, based on the respective polymer.Suitable comonomers include ethylene and α-olefins having 4 to 10 carbonatoms. It has proven particularly advantageous for the heat-sealableouter layer to contain one or more of propylene-ethylene copolymers,propylene-butylene copolymers, and propylene-ethylene-butyleneterpolymers and mixtures of these propylene polymers.

Particular preference for the heat-sealable olefin is given to randomethylene- propylene copolymers having an ethylene content of from 2 to10% by weight, preferably from 5 to 8% by weight, or randompropylene-1-butylene copolymers having a butylene content of from 4 to25% by weight, preferably from 10 to 20% by weight, in each case basedon the total weight of the copolymer, or randomethylene-propylene-1-butylene terpolymers having an ethylene content offrom 1 to 10% by weight, preferably from 2 to 6% by weight, and a1-butylene content of from 3 to 20% by weight, preferably from 8 to 10%by weight, in each case based on the total weight of the terpolymer, ora blend of an ethylene-propylene-1-butylene terpolymer and apropylene-1-butylene copolymer having an ethylene content of from 0.1 to7% by weight and a propylene content of from 50 to 90% by weight and a1-butylene content of from 10 to 40% by weight, in each case based onthe total weight of the polymer blend.

The multilayer film according to the invention is essentially free ofsilicone oil and comprises at least the above-described polypropylenebase layer and at least one heat-sealable outer layer which preferablycontains the above-described propylene polymers or mixtures thereof.Depending on its intended application, the multilayer film can have afurther outer layer on the opposite side. In a preferred embodiment, themultilayer film has three layers, where the outer layers applied may beidentical or different.

The thickness of the sealable outer layer(s) can be varied to achievethe desired properties and is generaly greater than 0.4 μm and ispreferably in the range from 0.6 to 4 μm, in particular from 0.8 to 2μm, where outer layers on both sides may have identical or differentthicknesses.

The overall thickness of the multilayer polyolefin film according to theinvention can vary within broad limits and depends on the intendedapplication. It is preferably from 5 to 70 μm, in particular from 10 to50 μm, the base layer making up from about 50 to 90% of the overall filmthickness.

It is essential to the invention that the multilayer film be essentiallyfree from silicone oil. It has been found that silicone oil-free filmsoffer a number of advantages, and, surprisingly, a very low coefficientof friction of less than 0.3 can still be achieved before and afterprocessing. Without silicone oil, the film can easily be corona-treatedwithout impairing the heat-sealability of the outer layer. The treatmentintensity readily can be measured without the necessity of removing thesilicone oil coating in an additional step. Without silicone oil, a barcode adheres well to the packaging film. Completely unexpectedly, thefilm of the present invention without silicone oil exhibits excellentprocessing properties.

Accordingly, the present films are essentially free of silicone oil, sothat these excellent properties are achieved. In a preferred embodimentof the present invention, the film contains a combination of a tertiaryaliphatic amine and an amide of a water-insoluble carboxylic acid having8 to 24 carbon atoms in its base layer and has SiO₂ in at least oneheat-sealable outer layer. Surprisingly, this additive combination makesit possible, without additional lubricants such as silicone oil oradditional polyolefinic resins (as in U.S. Pat. No. 4,117,193), toreduce the coefficient of friction of the polypropylene film to valuesof below 0.3 or below 0.2, generally from 0.12 to 0.3 or 0.12 to 0.2,thus providing a film which has the requisite surface-slipcharacteristics, but no longer has the disadvantages of the known filmshaving a low coefficient of friction. In particular the present film isreadily printable and has excellent values for gloss and haze. Thecoefficient of friction is measured by DIN 53 375 which is herebyincorporated by reference.

Carboxamides useful in the present invention include amides of awater-insoluble carboxylic acid having 8 to 24 carbon atoms or mixturesof these amides. Any such amides can be used with particular preferencegiven to erucamide, oleamide, stearamide and the like. It has provenparticularly advantageous to incorporate the amide only into the baselayer of the multilayer film, preferably in an amount of from 0.05 to0.3% by weight, particularly favorably in an amount of from 0.10 to 0.3,preferably from 0.10 to 0.25% by weight based on the weight of the baselayer.

Tertiary aliphatic amines useful in the base layer include any tertiaryaliphatic amines including compounds of the formula R₃ N, in which R isa fatty acid radical or a C₁₂ -C₁₈ -alkyl radical or ahydroxyl-substituted alkyl radical, where the radicals R may beidentical or different. Hydroxyl-substituted alkyl radicals arepreferably hydroxyethyl, hydroxypropyl or hydroxybutyl radicals.Particular preference is given to N,N-bis(2-hydroxyethyl)alkylamines.The tertiary aliphatic amines are particularly favorably employed in anamount of from 0.03 to 0.25% by weight, particularly advantageously inan amount of from 0.1 to 0.15% by weight, based on the weight of thebase layer.

The outer layer preferably contains from 0.1 or 0.2 to 0.5% by weight ofSiO₂. The SiO₂ has preferably been subjected to organic aftertreatmentcoating and has a mean particle diameter of from 2 to 6 μm, morepreferably, 3 to 5 μm, most preferably about 4 μm. The particlediameter: outer layer thickness ratio should be in the range from 3 to11, preferably from about 4 to 10, in particular from 6 to 8.

SiO₂ is preferably prepared by grinding silica gel and is a synthetic,highly porous, pure silicic acid which has a completely amorphousstructure, in contrast to crystalline silicic acids. The SiO₂ content isgenerally greater than 95%, in particular in the range from 98 to 99.5%.

According to the invention, the SiO₂ particles have preferably beensubjected to organic aftertreatment coating and have a coatingpreferably containing from 0.5 to 5% of an aliphatic carboxylic acid.Preferred aliphatic carboxylic acids are aliphatic hydroxydi- and-tricarboxylic acids and stearic acid. In general, the acids have two tofive, preferably two to three, hydroxyl groups. Preferred aliphaticcarboxylic acids are tartronic acid (hydroxymalonic acid), malic acid(monohydroxysuccinic acid), tartaric acid (dihydroxysuccinic acid) andcitric acid. Citric acid-containing coatings have proven veryparticularly advantageous. Due to the organic coating, the SiO₂particles are slightly acidic in aqueous solutions, such that the pH ofa 5% strength aqueous suspension is generally in the range from 3 to 5,preferably 4.

In addition to these selected additives, the multilayer film accordingto the invention may additionally contain other additives, such asneutralizers and/or stabilizers in effective amounts.

The stabilizers employed can be any of the conventional stabilizingcompounds for ethylene, propylene and other α-olefin polymers. Theamount thereof added is generally from 0.05 to 2% by weight. Phenolicstabilizers, alkali/alkaline earth metal stearates and/oralkali/alkaline earth metal carbonates are particularly suitable.

Phenolic stabilizers are preferred in an amount of from 0.1 to 0.6% byweight, in particular from 0.15 to 0.3% by weight, and with a molecularweight greater than 500 g/mol. Pentaerythrityl tetrakis3-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate! and1,3,5-trimethyl-2,4,6-tris(3,5-di-tert.-butyl-4-hydroxybenzyl)benzeneare particularly advantageous.

Neutralizers include any known neutralizers, but are preferably calciumstearate and/or calcium carbonate having a mean particle size of at most0.7 μm, an absolute particle size of less than 10 μm and a specificsurface area of at least 40 m² /g.

The invention furthermore relates to a process for the production of themultilayer film. Any desired process can be used, but the coextrusionprocess which is known per se, is generally used. This process involvescoextruding the melts corresponding to the individual layers of the filmthrough a flat-film die, taking off the resultant film on one or morerolls for solidification, subsequently biaxially stretching or orientingthe film, thermofixing the biaxially-stretched film and, if desired,corona-treating or flame-treating the film on the surface layer intendedfor corona treatment.

The biaxial stretching or orientation can be carried out simultaneouslyor consecutively, preference being given to consecutive biaxialstretching in which stretching is carried out first longitudinally (inthe machine direction) and then transversely (perpendicular to themachine direction).

As is usual in the coextrusion process, the polymer or polymer mixtureof the individual layers is first compressed and liquefied in anextruder, it being possible for the additives to be already contained inthe polymer or polymer mixture via conventional masterbatch processing.The additives also can be added directly to the polymer melt before theextrusion. The melts are then simultaneously forced through a flat-filmdie (slot die), and the extruded multilayer film is taken off on one ormore take-off rolls, during which it cools and solidifies.

The resultant film is then stretched longitudinally and transversely tothe extrusion direction, which results in an orientation of the moleculechains. The stretching in the longitudinal direction is preferably from4:1 to 7:1 and in the transverse direction is preferably from 8:1 to10:1. The longitudinal stretching is expediently carried out with theaid of two rolls running at different speeds corresponding to thedesired stretching ratio, and the transverse stretching is generalycarried out with the aid of an appropriate tenter frame.

The biaxial stretching of the film is followed by thermofixing (heattreatment) thereof, during which the film is kept at a temperature ofgenerally from 140° to 160° C., generally for from about 0.5 to 10 sec.The film is subsequently wound up in a conventional manner using awind-up unit.

It has proven particularly favorable to keep the take-off roll or rolls,which also cool and solidify the extruded film, at a temperature from50° to 110° C., preferably 80° to 110° C.

The temperatures at which the longitudinal and transverse stretching arecarried out may vary. In general, the longitudinal stretching ispreferably carried out at from 120° to 150° C., and the transversestretching at from 155° to 190° C.

If desired, as mentioned above, the biaxial stretching can be followedby corona or flame treatment of one or both surfaces of the film by oneof the known methods.

Corona treatment expediently involves passing the film between twoconductor elements serving as electrodes, with such a high voltage,usually an alternating voltage (from about 10 to 20 kV and 20 to 40kHz), being applied between the electrodes that spray or coronadischarges can occur. The spray or corona discharge causes the air abovethe film surface to ionize and react with the molecules of the filmsurface, forming polar intrusions in the essentially nonpolar polymermatrix.

For flame treatment with a polarized flame (cf. U.S. Pat. No.4,622,237), generally a direct electrical voltage is applied between aburner (negative pole) and a chill roll. The level of the appliedvoltage is generally from 500 to 3,000 V, preferably in the range from1,500 to 2,000 V. The applied voltage increases the acceleration of theionized atoms, which hit the polymer surface with greater kineticenergy. The chemical bonds within the polymer molecule become easier tobreak, and the formation of free radicals proceeds more rapidly. Thethermal load on the polymer during this procedure is substantially lessthan in the case of standard flame treatment, and films can be obtainedin which the heat-sealing properties of the treated side are even betterthan those of the untreated side.

The film according to the invention is distinguished by a very goodcoefficient of friction, generally less than 0.3, in combination withgood heat-sealing properties. Furthermore, the film can be printed verywell and on both sides. It thus has a combination of properties whichcannot readily be achieved simultaneously without disadvantageouslyaffecting other properties. In addition, the film has better gloss andhaze than known films.

The invention furthermore relates to a method for processing themultilayer film. Any desired processing can be applied to the film,preferably printing and/or lamination and/or coating and/or cuttingand/or embossing.

The film has particular advantages with respect to its processingproperties. It has been found that a low coefficient of friction of theouter layer alone does not reliably ensure good processing properties,and that the coefficient of friction of many films increases during saidfurther processing steps. Surprisingly, the films having the noveladditive combination have a coefficient of friction of the outer layerwhich is lower after processing than before, preferably less than 0.3after processing. This unexpected effect gives the film clearly superiorsurface-slip characteristics after processing than the films of theprior art. Neither the importance of the stability of the coefficient offriction during processing nor the targeted achievement ofprocessing-stable surface-slip characteristics were known hitherto.

The invention is now illustrated in greater detail with reference toworking examples.

EXAMPLE 1

A three-layer film having an overall thickness of 20 μm and an ABA layerstructure, i.e., a base layer surrounded by two identical outer layersA, was produced by coextrusion and subsequent stepwise orientation inthe longitudinal and transverse directions. Each of the outer layers hada thickness of 0.7 μm. Before rolling up, the film was corona-treated onboth sides. The surface tension of the film as a consequence of thistreatment was from 39 to 40 mN/m on both sides.

All layers contained 0.13% by weight of pentaerythrityl tetrakis4-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate! (®Irganox 1010) asstabilizer and 0.06% by weight of calcium stearate as neutralizer.

The base layer B essentially comprised a polypropylene homopolymerhaving an n-heptane-soluble content of 4% by weight and a melting pointof from 160 to 162° C. The melt flow index of the polypropylenehomopolymer was 3.2 g/10 min at 230° C. and a load of 21.6N (DIN 53735). The base layer contained 0.11% by weight ofN,N-bis(2-hydroxyethyl)-(C₁₂ -C₁₈)alkylamine (®Armostat 300) and 0.14%by weight of erucamide having a melting point of from 78° to 82° C.

The polyolefinic outer layers essentially comprised anethylene-propylene-1-butylene terpolymer having a content of 3% byweight of ethylene, 89.0% by weight of propylene and 8% by weight of1-butylene.

The outer layers contained 0.33% by weight of a silica having a meanparticle diameter of 4 μm which had been subjected to organicaftertreatment with an organic acid.

EXAMPLE 2

Example 1 was repeated, but the base layer contained 0.05% by weight ofthe tertiary aliphatic amine N,N-bis(2-hydroxyethyl)-(C₁₂-C₁₈)alkylamine and 0.17% by weight of erucamide.

EXAMPLE 3

Example 1 was repeated, but the polyolefinic outer layers essentiallycomprised a random ethylene-propylene-1-butylene terpolymer having acontent of 4% by weight of ethylene, 90% by weight of propylene and 6%by weight of 1-butylene. As in Example 1, the outer layers contained0.33% by weight of silicon dioxide which had been subjected to organicaftertreatment.

EXAMPLE 4

Example 1 was repeated, but the polyolefinic outer layers essentiallycomprised a random ethylene-propylene copolymer having a content of 4.5%by weight of ethylene and 95.5% by weight of propylene. As in Example 1,the outer layers contained 0.33% by weight of silicon dioxide which hadbeen subjected to organic aftertreatment.

COMPARATIVE EXAMPLE 1

Example 1 was repeated. As in Example 1, the outer layers essentiallycomprised a random ethylene-propylene-1-butylene terpolymer, but theouter layers did not contain any silicon dioxide.

COMPARATIVE EXAMPLE 2

Example 1 was repeated, but the base layer did not contain anyerucamide.

COMPARATIVE EXAMPLE 3

Example 1 was repeated, but the base layer did not contain anyN,N-bis(2-hydroxyethyl)-(C₁₂ -C₁₈)alkylamine.

COMPARATIVE EXAMPLE 4

Example 1 was repeated. The outer layers contained 0.33% by weight ofsilicon dioxide which had been subjected to organic aftertreatment and1.0% by weight of polydimethylsiloxane (silicone oil) having a kinematicviscosity of 30,000 mm² /s at 25° C.

COMPARATIVE EXAMPLE 5

A three-layer film having an ABA structure corresponding to Example 1 ofEP-A-0 182 463 which is hereby incorporated by reference was produced.The film had not been corona-treated.

COMPARATIVE EXAMPLE 6

Comparative Example 5 was repeated, but the film was subjected to coronatreatment on both sides before rolling up.

COMPARATIVE EXAMPLE 7

A three-layer film having an ABC structure corresponding to Example 1 ofEP-A-0 194 588 which is hereby incorporated by reference was produced.The first side, which contained no silicone oil, had been corona-treatedas described in EP-A-0 194 588.

The properties of the films of the examples and comparative examples areshown in Tables 1 and 2 below.

The raw materials and films were characterized using the followingmeasurement methods:

Melt Flow Index

The melt flow index was measured in accordance with DIN 53 735 at a loadof 21.6N and 230° C. or at a load of 50N and 190° C.

Melting Point

DSC measurement, maximum of the melting curve, heating rate 20° C./min.

Haze

The haze of the film was measured in accordance with ASTM-D 1003-52.

Gloss

The gloss was determined in accordance with DIN 67 530. The reflectorvalue was measured as an optical characteristic of the surface of afilm. In accordance with the standards ASTM-D 523-78 and ISO 2813, theangle of incidence was set at 20°. A light beam hits the planar testsurface at the set angle of incidence and is reflected or scatteredthereby. The light beams incident on the photoelectronic receiver areindicated as a proportional electrical quantity. The measurement valueis dimensionless and must be given together with the angle of incidence.

Seal Seam Strength

For the determination, two film strips 15 mm in width were laid one ontop of the other and sealed at 130° C. for 0.5 sec at a pressure of 10mm² (instrument: Brugger type NDS, sealing jaws heated on one side). Theseal seam strength was determined by the T-peel method.

Friction (Coefficient of Friction)

The friction was determined in accordance with DIN 53 375 on theheat-sealable layer.

Surface Tension

The surface tension was determined by the ink method (DIN 53 364).

Printability

The corona-treated films were printed 14 days after production(short-term assessment) and 6 months after production (long-termassessment). The ink adhesion was assessed by the adhesion-tape test. Ifthe adhesive tape was able to remove a little ink, the ink adhesion wasassessed as being moderate and if a significant amount of ink wasremoved, it was assessed as being poor.

Determination of the Minimum Heat-Sealing Temperature:

Heat-sealed samples (seal seam 20 mm×100 mm) are produced using theBrugger HSG/ET sealing unit by sealing a film at different temperatureswith the aid of two heated sealing jaws at a pressure of 10 N/cm² for0.5 sec. Test strips with a width of 15 mm are cut out of the sealedsamples. The T-seal seam strength, i.e., the force necessary to separatethe test strips, is determined using a tensile testing machine at a peelrate of 200 mm/min, during which the seal seam plane forms a right anglewith the direction of tension. The minimum heat-sealing temperature isthe temperature at which a seal seam strength of at least 0.5N/15 mm isachieved.

                                      TABLE 1                                     __________________________________________________________________________    Pretreatment intensity                                                        14 days after production         Coefficient of friction                       mN/m!        Printability                                                                          Coefficient of friction                                                                  (14 days after production                    Side 1  Side 2                                                                              Side 1                                                                            Side 2                                                                            14 days after production                                                                 after printing and lamination)               __________________________________________________________________________    E1 39   39    ++  ++  0.20       0.16                                         E2 39   39    ++  ++  0.20       0.15                                         E3 38   38    ++  ++  0.22       0.18                                         E4 39   39    ++  ++  0.20       0.17                                         CE1                                                                              39   39    ++  ++  film blocks >0.5                                                                         film blocks >0.5                             CE2                                                                              39   39    ++  ++  film blocks >0.5                                                                         film blocks >0.5                             CE3                                                                              38   39    ++  ++   0.26*      0.24*                                       CE4                                                                              --   --    --  --  film blocks 0.4-0.5                                                                      film blocks >0.5                             CE5                                                                              --   --    --  --  0.24       0.35                                         CE6                                                                              --   --    --  --  film blocks 0.4-0.5                                                                      film blocks >0.5                             CE7                                                                               38**                                                                              --    +-  --  0.28       0.5                                          __________________________________________________________________________     *The film is statically charged and is difficult to process.                  **The pretreatment intensity can only be measured after washing off the       silicone oil with nheptane.                                              

                                      TABLE 2                                     __________________________________________________________________________                   Minimum heat-sealing  Measurability                                   Gloss   temperature                                                                             Seal seam strength (HSE)                                                                  of the                                   Haze 4 Measurement                                                                           (15 N/cm.sup.2 ; 0.5 s)                                                                 (130° C.; 1.5 N/cm.sup.2 ;                                                         pretreatment                             layers angle 20°                                                                       °C.!                                                                             N/15 mm!   intensity                                 %!    Side 1                                                                            Side 2                                                                            Side 1/1                                                                           Side 2/2                                                                           Side 1/1                                                                            Side 2/2                                                                            Side 1                                                                            Side 2                               __________________________________________________________________________    E1 19  111 110 110  110  2.3   2.1   ++  ++                                   E2 20  110 109 110  110  2.1   1.9   ++  ++                                   E3 21  108 106 109  109  2.4   2.2   ++  ++                                   E4 21  109 107 122  122  1.3   1.1   ++  ++                                   CE1                                                                              18  113 112 109  109  2.4   2.2   ++  ++                                   CE2                                                                              19  112 112 110  110  2.3   2.1   ++  ++                                   CE3                                                                              19  110 109 110  109  2.1   2.0   ++  ++                                   CE4                                                                              22  107 106 no sealing                                                                         no sealing                                                                         no sealing                                                                          no sealing                                                                          -+  -+                                   CE5                                                                              22  108 108 122  120  1.4   1.2   --  --                                   CE6                                                                              22  109 108 no sealing                                                                         no sealing                                                                         no sealing                                                                          no sealing                                                                          -+  -+                                   CE7                                                                              21  107 106 124  122  1.3   1.1   -+  --                                   __________________________________________________________________________

What is claimed is:
 1. A process for the production of a multilayerpolyolefin film comprising a base layer including a propylene polymer, atertiary aliphatic amine, and an amide of a water-insoluble carboxylicacid having 8 to 24 carbon atoms, and a heat-sealable outer layercomprising a heat-sealable olefin polymer and SiO₂ particles, whereinthe multilayer film is essentially free from silicone oil, the processcomprisingextruding melts corresponding to individual layers of the filmthrough a flat-film die, taking-off the coextruded film via a take-offroll, biaxially stretching the film, thermofixing thebiaxially-stretched film, optionally flame- or corona-treating the film,and winding up the film.
 2. A process as claimed in claim 1, wherein theSiO₂ particles have an organic coating and have a mean particle diameterof from 2 to 6 μm.
 3. A process as claimed in claim 2, wherein the meanparticle diameter is about 4 μm.
 4. A process as claimed in claim 1,wherein the base layer contains from 0.03 to 0.25% by weight based onthe weight of the base layer of the tertiary aliphatic amine and from0.05 to 0.3% by weight based on the weight of the base layer of theamide, and wherein the outer layer contains from 0.1 to 0.5% by weightof SiO₂, based on the weight of the outer layer.
 5. A process as claimedin claim 1, wherein the heat-sealable outer layer has a coefficient offriction of less than 0.3 after processing, by one or more of printing,laminating, coating, cutting or embossing.
 6. A process as claimed inclaim 5, wherein said outer layer has a coefficient of friction of from0.12 to 0.3 after processing.
 7. A process as claimed in claim 1,wherein there is an outer layer on each side of the base film, whereinthe outer layers are the same or different.
 8. A process as claimed inclaim 1, wherein the heat-sealable outer layer has a thickness from 0.6to 4 μm.
 9. A process as claimed in claim 1, wherein the overallthickness of the film is from 5 to 70 μm.
 10. A process as claimed inclaim 1, wherein the SiO₂ particles have a coating of aliphaticcarboxylic acid.
 11. A process as claimed in claim 10, wherein the SiO₂particles are coated with 0.5 to 5% by weight of aliphatic carboxylicacid, based on the weight of the particles.
 12. A process as claimed inclaim 1, wherein the base layer consists essentially of the propylenepolymer, the amine, and the amide.
 13. A process as claimed in claim 12,wherein the base layer further consists essentially of at least oneadditive selected from the group consisting of a neutralizer and astabilizer.
 14. A process as claimed in claim 1, wherein theheat-sealable outer layer comprises a copolymer of propylene orterpolymer of propylene.
 15. A process as claimed in claim 1, whereinthe take-off roll has a temperature of from 50° to 110° C.
 16. A processas claimed in claim 1, wherein the biaxially stretching compriseslongitudinal stretching at a ratio of from 4:1 to 7:1 and transversestretching at a ratio of from 8:1 to 10:1.
 17. A process as claimed inclaim 1, wherein the thermofixing comprises keeping the film at atemperature of 140° to 160° C. for about 0.5 to 10 seconds.
 18. Aprocess as claimed in claim 1, wherein the biaxially stretchingcomprises longitudinal stretching at a temperature of 120° to 150° C.and transverse stretching at a temperature of 155° to 190° C.